1. Field of the Invention
The present invention relates in general to a flexible printed circuit and pertains, more particularly, to a flexible printed circuit that supports electronic microcircuits for use in a card whose thickness meets the International Standard Organization (ISO) standard for credit cards.
2. Description of Prior Art
A card with electronic microcircuits is a single piece or multilayer rectangular card, made of plastic material. The card has contacts on an exterior surface for connection of the electronic microcircuits (integrated circuit chip) with a device for the processing of the card. These microcircuits may be designed for a variety of different uses, such as, for example, bank debit and credit transactions, the allocation of telephone systems, and confidential access in a protected environment. These cards generally have one or more complex processing and/or memory circuits, depending on the use for which they are intended. In practice, the electronic microcircuits may be comprised of at least one silicon wafer, typically referred to as an integrated circuit or circuit chip.
One form of a card construction is one in which the card is provided with a cavity in which is supported a printed circuit which carries the integrated circuit and, furthermore, carries card contacts. The printed circuit is typically made from a thin sheet of flexible material, such as polyester, epoxy glass, or as well, a plastic material which has sufficient flexibility. Given the fact that the printed circuit, equipped with contacts and an integrated circuit, is to be contained in a card whose thickness, according to standards, is relatively thin, such as 0.762 mn.+-.10%, the sheet for the printed circuit is to be thin, on the order of, for example, 130 micrometers. The flexible material for the sheet and its thinness realize an element which is compatible with the flexibility required by the ISO for cards of the credit card type.
The flexible printed circuit, on one side thereof, carries the integrated circuit, and on the other side thereof carries the card contacts. The contacts are, in this manner, placed at a position remote from the integrated circuit and are connected to terminals of the integrated circuit by leads which are disposed on one side of the flexible printed circuit sheet. For reasons of convenience, the side of the sheet on which the contacts are accessible is termed the upper or external side of the sheet, and the other side of the sheet is termed the lower or internal side thereof In addition, the axis which connects the card contacts and the integrated circuit is termed the circuit axis.
In accordance with one method of fabricating the flexible printed circuit, the leads and the contacts with which they are connected are disposed on the upper side of the sheet. The terminals of the integrated circuit are soldered to the corresponding ends of the leads, which are arranged overhanging a recessed opening in the sheet. The ends of the leads may be formed in a manner to facilitate their connection with the corresponding terminals on the integrated circuit. A flexible printed circuit constructed in this manner is thin due to the fact that the thickness of the integrated circuit is not added to the thickness of the sheet. This is accomplished primarily by disposing the integrated circuit in a through opening of the sheet.
According to another method of production of the flexible printed circuit, the side of the integrated circuit which is opposite to that which carries the terminals of the integrated circuit is the one which is mounted on the lower side of the sheet, and the terminals of the integrated circuit are therefore connected to the matching ends of the sheet leads by lead wires. This technique employs "wire bonding".
In the production of the flexible printed circuit, the circuit leads may be covered with a protective coating. Alternatively, it is advantageous to spread this coating over the entire side of the card, especially when the leads for the printed circuit run the risk of being located in proximity to, or in contact with, a magnetic strip on the card. It is difficult to lay the magnetic strip if it is not mounted on a homogeneous material. When the coating is employed, of course, in that case openings are provided in the coating in order to provide access to the contacts.
One variant method of production of the printed circuit involves constructing the leads on the lower side of the flexible sheet. The integrated circuit is soldered to the matching lead ends, while, on the other end, the contacts are accessible in recessed openings in the sheet. This variant does not require the use of a coating, but it has the problem of adding to the thickness of the sheet and of the integrated circuit.
Moreover, the staggering of the contacts with respect to the integrated circuit may be preferred for several reasons, especially the need to meet the standards for placement of the contacts on the center line of the card, near one of the shorter edges. Due to this arrangement, the integrated circuit may remain in a corner of the card where tortional and bending stresses are clearly less than at the area of the contacts. ISO standards also dictate that the contacts are to be six or eight in number, distributed in an equal manner in two rows parallel to one of the shorter edges of the card. In accordance with another ISO arrangement, the contacts should have a predetermined minimum surface, rectangular in shape. The leads ordinarily run from the contacts and are connected in a direct and compact fashion with the corresponding terminals of the integrated circuit.
In order to meet international standards, the card has to be capable of enduring, without damage, a given number of flexures, which are performed following one or the other of the center lines of the card. Given this requirement, it has been determined, that printed circuits with staggered contacts designed in the standard manner are generally deficient in meeting minimum standards. As a general rule, the flexures which take place in the direction of the circuit axis are the most damaging. This damage is characterized by breaks in the leads. Studies have, therefore, been undertaken in order to find the best metallurgy and the best method of production for leads. The results obtained have been determined to be unsatisfactory with regard to both quality and reliability.
Accordingly, it is an object of the present invention to provide a flexible printed circuit with the means to ensure the quality of the electrical connections between the contacts and the circuit terminals in a lasting manner, despite the stress created by flexing of the printed circuit.